Nonlinear optical crystals such as LiNbO3, LiTaO3 and the like have been preferably used as materials of elements used for wavelength conversion such as second harmonic generation (SHG), optical parametric oscillation, optical parametric generation (including amplification), difference frequency generation, sum-frequency generation and the like.
As a means for satisfying the phase matching conditions for such wavelength conversion, quasi-phase matching (QPM) including formation of a periodically-poled structure (hereinafter to be also referred to as a “poled structure”) on a nonlinear optical crystal has been actively conducted in recent years. The quasi-phase matching is described in detail in, for example, a publication, Optical Second Harmonic Generation and Polarization Reversal, Kurimura, Solid-State Physics, 29(1994) (75–82) and the like.
As shown in FIG. 2, a poled structure element (wavelength conversion element) is an element wherein the polarizational direction (z direction in the Figure) of a nonlinear optical crystal 10 is periodically reversed (i.e., nonlinear optical constant has been modulated) to achieve a high wavelength conversion efficiency, and the nonlinear optical constant to be utilized is exclusively d33, because its value is the highest. That the nonlinear optical constant d33 can be used is the advantageous aspect of the quasi-phase matching method.
In conventional poled structure elements, what is called a z plate (crystal substrate processed to make the substrate surface perpendicular to the z-axis of optical crystal) is used and a polarization reversal period utilizing the nonlinear optical constant d33 is formed. As shown in FIG. 2, the polarized light direction of an incident light L10 and the polarized light direction of a wavelength converted outgoing light L20 are both parallel to the z-axis of the nonlinear optical crystal. In this way, only the utilization of the nonlinear optical constant d33 has been conventionally taken note of and group velocity matching of the incident light and the outgoing light has not been considered at all.
For wavelength conversion, a light having a pulse train (pulsed light) is sometimes used as an incident light. Examples thereof include conversion of, a pulsed light having a wavelength of 1.5 μm to a pulsed light having a wavelength of 0.78 μm by SHG, computing (e.g., sampling and gating for time-division multiplex communication, channel conversion and routing for wavelength multiplex communication) of a pulsed light having a wavelength of 1.5 μm and a pulsed light having a wavelength of 0.78 μm, and the like.
However, when the present inventors studied wavelength conversion behavior of the above-mentioned conventional poled structure element, it was found that, when a pulsed light is handled, the incident light and the outgoing light are separated in space and in time, along with the propagation of the light, due to a difference in the group velocity between the incident light and the outgoing light, and as a result, the following various problems such as those described below occur.
For wavelength conversion of continuous light, for example, since incident light exists over the entire length of the element, the conversion efficiency and computing efficiency are expected to be improved by prolongation of the element length. In contrast, when a short pulsed light is to be handled, such as pulse-number 1 Tbit/sec or above (=pulse width 1 ps or below), the incident light and the outgoing light are separated due to a difference in the group velocity between them, posing a problem in that the conversion efficiency and computing efficiency are not improved even if the element length is prolonged. A problem also occurs in that, as a result of wavelength conversion, the pulse width of the outgoing light is extended depending on the difference in the group velocity, making retention of the pulse shape difficult, and accurate computing results cannot be obtained. In some cases, a problem also occurs in that pulses before and behind in the pulse train interfere with each other and produce serious errors in computing. Such problems are clearly recognized when the pulse width is shorter and the pulse-number is higher.
In addition, since this difference in the group velocity depends on the wavelength of the incident light and the wavelength of the outgoing light (converted light), it becomes a factor that limits the wavelength band of the incident light. In a 1.5 μm band wavelength variable light source using 0.78 μm wavelength as an exciting-light source, moreover, the realizable wavelength band is limited due to the group velocity dispersion.